1. Field of the Invention
The present invention relates to a zoom lens system, and more particularly, to a zoom lens system for use in a copying apparatus where zooming is performed at a constant conjugate distance.
2. Description of the Prior Art
In a conventionally-used copying apparatus, in order to vary the magnification, the conjugate distance is varied by moving a mirror as a projecting lens moves. For such a copying apparatus, since the mirror is moved in order to vary the conjugate distance, a mechanism for moving the mirror is required. For this reason, the copying apparatus is disadvantageous in reducing size and cost.
In resent years, zoom lens systems have been suggested to vary the magnification in a copying apparatus.
U.S. Pat. No. 4,805,999 discloses a zoom lens system for use in a copying apparatus including eight lens elements and having an F number of 8 to 6.3, a half angle of view of 19.2.degree. and a magnification of 0.5.times. to 2.0.times.. This zoom lens, although having a wide zoom range, has a disadvantage that its cost is high and that it is large in size, since it has a large number of lens elements.
U.S. Pat. No. 4,813,773 discloses a zoom lens system for use in a copying apparatus including five or six lens elements and having an F number of 7, a half angle of view of 17.degree. to 21.degree. and a magnification of 0.64.times. to 1.42.times.. This zoom lens system has a disadvantage that it is long in total length and large in size.